This invention relates to compositions and methods used in completing oil and gas wells, especially wells having deviated or horizontal boreholes.
Hydrocarbon fluids such as oil and natural gas are obtained from a subterranean geologic formation, i.e., a reservoir, by drilling a well that penetrates the hydrocarbon-bearing formation. Once a wellbore has been drilled, the well must be completed; a completion involves the design, selection, and installation of equipment and materials in or around the wellbore for conveying, pumping, or controlling the production or injection of fluids. After the well has been completed, production of oil and gas can begin.
In deep wells, reliability of the sand face completion is very important, due to the prohibitively high cost of intervention. Further, many such wells are completed open hole. Therefore, gravel packing of open-hole horizontal wells is increasingly becoming a standard practice in the deep-water, sub-sea completion environment. The gravel packing process involves pumping into the well a carrier fluid that contains the necessary amount of gravel. Consequently, gravel is deposited adjacent an open hole, for instance at one end of the wellbore, where it serves to prevent sand from flowing into the wellbore.
Proper selection of the carrier fluid is essential. Ideally, the carrier fluid shall not cause any permeability reduction of the formation. When viscous fluids are used, carrier fluid must also have sufficient viscosity to suspend and carry the gravel during placement. Aqueous-base fluids can be tailored to be compatible with most formations simply by including salts such as potassium chloride or ammonium chloride. Consequently, to date, the convention in gravel-packing such horizontal wells has been water packing or shunt-packing with water-based viscous fluids. A carrier fluid is thus typically a brine comprising a gelling agent such as hydroxyethylcellulose (HEC), xanthan or a viscoelastic surfactant; breakers to minimize the pressure required to move the fluid back to the wellbore.
However, aqueous gravel pack fluids are poorly compatible with oil-based drilling fluids. Typically, the water content of the oil-base drilling fluid will tend to increase upon contact with the gravel pack fluid resulting in an increase of the fluid viscosity and in the formation of sludge. So, to ensure compatibility and improve wellbore cleanup, the section of the well to be gravel-packed is drilled with a water-based drilling fluid so that the drilling fluid residue left on the formation adjacent to the borehole in the form of a filter cake be also water-based. Then, the filter cake may be wholly or partly dissolved by acid wash solution, or as it has been proposed in the British Patent Application 2 338 254 and in the pending U.S. application Ser. No. 09/552,853, Continuation-in-Part of U.S. Pat. No. 6,140,277, by an aqueous wash composition comprising water and an effective amount of cationic salts of polyaminocarboxylic acids at neutral or acidic pH, including for instance ethylenediaminetetraacetic acid (EDTA).
Nevertheless, the use of a water-based drilling fluid is not always suitable or the best recommended practice. Indeed, oil-base drilling fluids provide shale inhibition, lubrication, gauge hole, and higher rates of penetration lubricity and deeper bit penetration and therefore, may often be preferred over water-base fluids. In the art of drilling fluid, the term xe2x80x9coil basedxe2x80x9d fluids is used to designate fluids having a continuous phase based on synthetic or non-synthetic oil and eventually, an aqueous phase dispersed in the oil phase.
Even though a compromise between the advantages of oil based drilling mud and water based gravel pack carrier fluids can be achieved by drilling the top hole with oil based mud, particularly in shaly zones, then switching to a water based reservoir drilling fluid for the horizontal reservoir section, this fluid swap, from oil based to water based drilling fluid, complicates fluid management on the rig site, increases the risk of fluid incompatibility, and increases overall completion costs.
An alternative method recently practiced offshore West Africa and the Gulf of Mexico, has been to keep drilling with the oil based reservoir drilling fluid even into the horizontal well section, and subsequently displace the oil based fluid with water based fluid in preparation for a water based gravel-pack carrier fluid. This practice can be less expensive than switching to water based reservoir drilling fluid for the horizontal section. However, this technique is still more complex and expensive than desirable and can lead to problems installing the sand exclusion hardware (screens).
There is a need for gravel pack carrier fluids and gravel packing methods that avoid incompatibility issues and eliminate the need for switching from an oil based drilling fluid to a water based drilling fluid, or the need for displacements to water based-fluids.
One aspect of the invention is a method of gravel packing a hole in a subterranean formation, a part of the formation adjacent to the hole having a filter cake coated thereon, said filter cake comprising an emulsion, said method comprising injecting into the wellbore a composition that comprises gravel and a carrier fluid having an aqueous phase and further comprising the step of breaking or inverting the filter cake emulsion.
Where the carrier fluid comprises an emulsion, said emulsion may be simultaneously broken during the step of breaking of inverting the filter cake emulsion. By simultaneously, it is understood that the breaking occurs during a single treatment, not necessarily rigorously at the same time. For instance, the carrier fluid may comprise a filter-cake breaking additive that is released once the emulsion of the carrier fluid is broken. Consequently, the step of breaking or inverting the filter cake may be completely independent of the step of injecting the gravel pack fluid (injection of a breaking composition) or being a direct consequence of the step of injecting the gravel pack fluid.
According to a preferred embodiment, the aqueous phase of the carrier fluid has a pH that is outside the pH range of stability of the filter cake emulsion. For instance, where the filter cake comprises a water-in-oil emulsion that is stable in a first pH range above about 7 or 8, the aqueous phase of the carrier fluid can include a mineral or organic acid. The aqueous liquid is present in the carrier fluid in an amount effective to invert the emulsion (or residue thereof) in the filter cake when the carrier fluid comes into contact with the filter cake. As a result of injecting the gravel pack composition, gravel is deposited at or adjacent to the aperture in the wellbore and usually at least a portion of the filter cake is removed. xe2x80x9cRemovalxe2x80x9d of filter cake herein means dissolving the filter cake or breaking it apart or loose from the formation, such that fluid can flow more readily.
The filter cake typically contains bridging/weighting agents such as calcium carbonate, barite, salt, or manganese oxide, among others. When the emulsion in the oil-based drilling fluid filter cake inverts, those bridging/weighting agents then come into contact with an increased volume of aqueous fluid from the gravel pack carrier fluid, and as a result they will entirely or largely dissolve. Preferably, the filter cake and/or the carrier fluid also contain one or more surfactants to enhance this wetting (and thus the dissolution) of the bridging/wetting agents. This enhances the overall filter cake cleanup process. In one preferred embodiment, the pH of the carrier fluid is adjusted with a pH-modifying agent that also enhances the solubility of drilling fluid bridging/weighting agents that are present in the filter-cake.
Optionally, the filter cake and the carrier fluid can be broken simultaneously by injecting into the wellbore a breaker fluid composition that comprises at least one of a surfactant and a solvent that is capable of breaking or dissolving both the filter cake and the carrier fluid. The solvent is preferably a mutual solvent that is capable of dissolving components that would otherwise be insoluble. The surfactant(s) preferably is chosen to enhance wetting of bridging/wetting agents in the filter cake. Preferably, the breaker fluid is also capable of dissolving at least some portion of the filter cake.
Suitable pH modifying agents include mineral acids (such as hydrochloric acid), organic acids (such as formic acid, acetic acid, or citric acid), and chelating agents, in particular cationic salts of polyaminocarboxylic acids chelating agents suitable typically using at neutral or mild pH, ranging from 3.5 to 8.0. Examples of chelating agents include aqueous solutions comprising di-cationic salts (and preferably di-potassium salts) of ethylenediaminetetraacetic acid (EDTA), cyclohexylene dinitrilo tetraacetic acid (CDTA), [Ethylenebis(oxyethylenenitrilo)]tetraacetic acid (EGTA, also known as Ethyleneglycol-bis-(beta-aminoethyl ether) N,Nxe2x80x2-tetraacetic acid) and [[(Carboxymethyl)imino]-bis(ethylenenitrilo)]-tetra-acetic acid (DTPA, also known as Diethylenetriaminepenta-acetic acid), hydroxyethylethylenediaminetriacetic acid (HEDTA). Hydroxyethyliminodiacetic acid (HEIDA), that has been proposed in U.S. Pat. No. 5,972,868, as a chelant for the removal of alkaline earth scale in downhole equipment can also be used, in a form of free acid or salt. Some of the above mentioned chelants can also be used in an alkaline pH range and thus could be used with filter cake from a drilling fluid emulsion that is stable at low pH and unstable at high pH.
The carrier fluid may be a pure aqueous fluid but is most preferably an emulsion. The carrier fluid can comprise between about 50 and 90% by volume aqueous phase. Preferably, about 50-70% by volume of the carrier fluid is aqueous phase. The water phase may constitute the external continuous either phase of the emulsion (oil-in-water emulsion) or the discontinuous phase (water-in-oil emulsion). Water-in-oil emulsions are preferred since they are fully compatible with oil-base filter cakes. It is believedxe2x80x94though it is understood that the invention should not be limited to this theory, that along time, the water droplets will coalesce so that the emulsion becomes non stable and the water phase will enter into contact with the filter cake emulsion, and finally breaks it.
Where the gravel pack carrier fluid has an external water phase (or its only liquid phase is a water phase), it preferably further comprises a viscosity enhancing agent, for instance a viscoelastic agent gelled in a solution of a chelating agent (such as EDTA). It this latter case, it is recommended to first displace the oil-base drilling mud with displacement fluid that has a composition compatible with the gravel pack carrier fluid and that does not comprise pH modifying agent or a chelating agent; then, the gravel pack carrier fluid will be injected.
Injection of this gravel pack composition into the wellbore results in simultaneous deposition of gravel at or adjacent to the open hole and removal of sufficient filter cake to establish a fluid flow path between the wellbore and the formation. This method is especially useful in wellbores that are drilled with an oil based reservoir drilling fluid and completed open hole, particularly in horizontal boreholes. The present invention has several benefits as compared to prior art gravel packing methods and carrier fluids, including reduced cost, improved fluid management practices, and increased productivity and/or reduced risk of future interventions, by mitigating against the risk of sand face failure.